Bimetallic cylinders have been employed in the plastic extrusion and injection molding industries for many years. For purposes of this specification, a bimetallic cylinder is one which includes a cylindrical body and a lining of a hard and wear and corrosion resistant metal or alloy. Bimetallic cylinders have become widely accepted because in extrusion or injection molding a screw forces a hot, molten plastic through a special shaped die and typically the plastics include abrasive filler materials. As a result of the high temperatures employed in the process, the abrasive fillers and the action of the screw itself, standard steel cylinders quickly wore or became corroded.
Kormann and Hirsch were pioneers in the development of of bimetallic cylinders in the early 1930's and some of their work is described in U.S. Pat. Nos. 2,049,913 and 2,046,914, each issued July 7, 1936. The process for preparing bimetallic cylinders described in these patents comprises placing a steel cylinder in a horizontal position and loading the cylinder with a quantity of an alloy having a melting point less than the cylinder itself. The alloy has greater wear and corrosion resistance properties than the cylinder. The ends of the cylinder are closed by welding caps over them and the cylinder is gradually heated above the melting point of the lining material. The cylinder is rapidly spun about its axis to centrifugally spread the melted lining material over the inner surface of the cylinder. The cylinder is then cooled gradually, the end caps are removed and laths and hones are used to finish the inside surface to the desired diameter and smoothness.
A variety of different alloys have been used for the lining of bimetallic cylinders. Ferrous alloys were used by Kormann and Hirsch in the aforementioned U.S. Pat. No. 2,049,913 and by Saltzman in his U.S. Pat. No. 3,658,515 issued Apr. 25, 1972. The ferrous materials have a hardness in the approximate range of 58-64 Rockwell C and exhibit good wear resistance when used in bimetallic cylinders. However, ferrous alloys are not particuarly well suited for use in corrosive environments and attempts to improve corrosion resistance in ferrous alloys usually led to reduced wear resistance.
More recently, non-ferrous lining alloys have been developed for use in bimetallic cylinders. One type includes about 40% nickel, 45% colbalt, 8% chronium and 3% boron in addition to minor amounts of carbon, manganese, silicon, etc. Still more recently carbide particles have been mixed with the non-ferrous base metal to improve performance. One particularly useful carbide alloy is described in U.S. Pat. No. 4,089,466 issued to the present inventors on May 16, 1978. The latter contains 10-35% tantalum carbide in a matrix alloy comprising 0.16-0.35% carbon, 28.5-34.6 nickel, 9.5-7.5% chromium and 28.5-42.0% colbalt (all weights being expressed in weight percent). Still more recently, an improved alloy has been marketed which includes a mixture of at least two carbides admixed with a nickel-cobalt base alloy.
While the state of the art is well advanced in terms of lining alloys and the design thereof for particular end use applications, little attention has been directed to the performance of the overall cylinder. Moreover, in recent years bimetallic cylinders have been used in a variety of applications other than injection molding and extrusion. For example, such cylinders are rapidly replacing unlined or chrome plated stainless steel cylinders in the food and drink processing industries.
The introduction of bimetallic cylinders for such end uses has, however, led to additional problems which have not been overcome prior to the present invention. In many food applications, the cylinders are subjected to daily acid washings, e.g. for 45 minutes at 175.degree. F. Also, in some applications, the entire cylinder may be surrounded by a cooling fluid or heating liquid and exterior corrosion has resulted. Corrosion is most noticeable and most troublesome at the ends of the cylinder where they may be clamped or supported in the particular apparatus in which they are used. Such corrosion problems have hindered the commercial acceptance of bimetallic cylinders in new industries.